The purpose of associating file extensions with MIME types on a web server is to enable the server to tell a browser specifically what kind of file is being downloaded, without relying on the browser to know every possible file extension. For example, you would tell your web server all of the extensions that specify JPEG images on your server (these could be ".jpg", ".jpeg", and ".jpe") and associate these with a MIME type of "image/jpeg". This allows the browser to identify the file type and determine how to display files with the JPEG extensions.